The present invention relates to an exhaust emission control device for Diesel engines and more particularly to an exhaust emission control device for Diesel engines in which particulates in the exhaust gas from a Diesel engine are collected on a trap and oxidized using a burner to regenerate the trap.
Among systems for reducing particulates in the exhaust gas employed by exhaust emission control devices for Diesel engines, including the mechanical particulate trapping system and the electrostatic precipitation system, such a system is general that particulates are collected on a trap and oxidized. One of methods of oxidizing particulates employs a burner, and such a device has hitherto been proposed that a trap is fitted in an exhaust pipe connected to the exhaust manifold of a Diesel engine, and a burner is provided on the upstream side of the trap in the exhaust pipe to oxidize particulates collected on the trap by means of flames jetted out into the exhaust pipe. The device of this type has been disclosed in W. R. Wade et al, "Diesel Particulate Trap Regeneration Techniques", SAE Paper No. 810118, February, 1981.
The oxidation rate constant k of particulates is expressed by the Arrhenius' equation: k=A exp(-.DELTA.E/RT). It will be clear from the equation that the oxidation rate depends upon only temperature. Accordingly, in order to oxidize particulates with high efficiency, it is necessary to rapidly raise the atmospheric temperature of the collected particulates. In the conventional device having the above-described construction, however, since the flames from the burner are applied only from the trap inlet front surface, the heat-receiving area of the trap is small, so that it is impossible to obtain a uniform temperature rise throughout the trap. Moreover, since the flames from the burner are directly jetted out into the exhaust gas, the combustion condition of the burner is affected by the flow rate of the exhaust gas, the exhaust pressure, the amount of O.sub.2 in the exhaust gas and so forth, which largely vary according to the engine operating conditions. Therefore, it is extremely difficult to effect control so that the burner will perform a complete combustion at all times.